压扁型烧结热管表面局部凹坑缺陷形成机制分析

Formation Mechanism of Local Pits on Surface of Flattened Sintered Heat Pipes

为了适应电子产品轻薄化、小型化的发展需求,压扁型薄壁烧结热管由于其优异的传热性能得到广泛应用,5G技术的应用导致电子产品内部热流密度的大幅度提高,对热管散热性能提出了更高的要求。烧结热管进行压扁工序时容易出现表面局部凹坑缺陷,这种表面凹坑会降低热管的表面质量,降低热管传热性能。本文通过光学显微镜(OM)、电子背散射衍射(EBSD)和白光干涉试验对热管压扁表面局部凹坑位置进行宏观形貌和微观组织分析,并且采用万能拉伸实验机对烧结热管进行力学性能测试。结果表明:烧结后热管内部再结晶晶粒明显长大,局部异常长大晶粒尺寸达到2.17 mm,在壁厚方向上仅为一层晶粒,呈现“饼状”的长大方式;基体形成强烈的再结晶织构,并且分布着具有异常取向的条带状退火孪晶;异常长大晶粒和退火孪晶对热管组织造成的尺寸不均匀和取向不均匀是导致烧结热管局部凹坑的主要原因。同时,异常长大的晶粒会导致烧结热管的屈服强度、抗拉强度和延伸率的下降。

With the advancement of electronic information technology,heat dissipation is a more prominent problem in the application of electronic products. In order to meet the development needs of light,thin and miniaturized electronic products,flattened thinwalled sintered heat pipes have been widely used due to their excellent heat transfer performance. The application of 5G technology in the internal heat flow density of electronic products is excellent at present. And it has led to a substantial increase. Moreover,it has set higher requirements for heat pipe in heat dissipation performance. The sintered heat pipe is prone to partial surface pit defects during the flattening process. Such surface pits are detrimental to the surface quality of the heat pipe. The air layer with a small thermal conductivity might remain in the pit. The heat transfer performance of the heat pipe would be reduced. The sintered heat pipe needs to be carried out at a high temperature of 950 ℃ during the processing. The original copper tube is sintered at a high temperature. And the process of recovery,recrystallization and secondary recrystallization occured. The surface quality of the heat pipe is affected by the abnormal growth of recrystallized grains. Through optical metallography,electron backscattered diffraction(EBSD)and white light interference experiments,the macroscopic morphology and microstructure of the local pits on the flattened surface of the heat pipe were analyzed,and the pit defects on the surface of the ultrathin sintered heat pipe after flattening were investigated. The universal tensile testing machine was used to test the mechanical properties of the sintered heat pipe. The results showed that after sintering the recrystallized grains inside the heat pipe grew significantly,and the size of the locally abnormally grown grains reached 2.17 mm,which was only a layer of grains in the wall thickness direction,showing a "cake-like" growth method. The pit-free sample heat pipe had smaller size recrystallized grains,and when the grain size became small,it had a strong ability to coordinate deformation. There was no pit defect during the flattening process of the heat pipe;the flattened surface was concave by white light interference observation of the pits,indicating that the flattened surface was rough,and the pits showed an elliptical shape elongated in the axial direction. In the early stage of flattening,the metal on the upper part of the round tube flowed to both sides. With the increase of the reduction,the contact area between the movable mold and the heat pipe gradually increased. The friction between the movable mold and the tube prevented the metal on the upper part of the tube flow to both sides,the metal in the middle of the flat surface was concave due to no support from below. The comparison of the macroscopic morphology photos and the microstructure showed that the pit morphology was related to the shape of abnormally grown grains. According to the test results of EBSD,the matrix formed a strong recrystallized texture,and there were strips of annealing twins with anomalous orientation. The main texture type of the sintered heat pipe was Goss texture with a small amount of interspersed copper texture. The main matrix texture of the heat pipe was the Goss texture with a larger Schmidt factor,and the twins were the copper texture with a smaller Schmidt factor. The interaction between abnormally growning grains and annealing twins during the flattening process caused uneven forced on each grain,and the mechanical properties of grains with different orientations were different. The microscopic force resulted in some larger and easily deformed Goss-oriented grains. The microscopic force caused some of the larger and easily deformable Goss-oriented grains to deformation to a greater degree,and hard-oriented grains were more difficult to deform. They showed pit defected on a macroscopic scale,and the position of pits generally appeared in the middle of the flat surface. The uneven grain size and uneven orientation produced during the sintering process were the main reasons for the local pits of the sintered heat pipe.